This invention relates to apparatus for reducing the axial endplay of a motor armature shaft and particularly to such apparatus which minimizes the internally generated noise which is transmitted to the motor case for radiation to the surrounding environment.
Armature shafts in electric motors are sometimes subjected to axial forces which may cause them to move axially back and forth noisily between whatever stops are provided by the motor case unless they are restrained in some manner. For example, in a windshield wiper motor, such axial motion is often caused by load reversals as the windshield wiper blade changes wipe direction.
Among the various means for reducing the axial endplay of such motor armature shafts is the inclusion of a thrust bearing arrangement in which a knob of good bearing material or a steel ball is pressed into the shaft end to bear against a thrust plate which is made of a compatible bearing material. In some cases in the prior art, the arrangement is made somewhat resilient to take up any axial endplay, as in the resilient elongated arms of the sheet metal plates in the structure shown in the U.S. Pat. No. 2,030,193 to Arnold or the resilient nylon disk of the arrangement in the U.S. Pat. No. 2,904,709 to Lautner. Sometimes, as in the U.S. Pat. No. 3,549,218, to Cagnon et al a thrust member is supported directly by the motor case.
However, it is not only desirable that the apparatus used to take endplay of a motor armature shaft have superior bearing qualities. It is further desirable, in many applications, that the arrangement minimize the transmission of audible noise generated in the armature or between the armature and thrust bearing surface during motor operation to the motor case for radiation to the motor environment. In general, the most favorable bearing materials such as steel or Vespel.RTM. are very hard materials and therefore good noise conductors; and those resilient materials which have lessened noise transmissibility are generally not suitable for bearing use. Even the addition of a resilient backing or similar support for a hard bearing material does not necessarily decrease the noise transmission therethrough, since the thrust load generally places the resilient material in compression and thereby increases its noise transmissibility.